The governor said he hopes people will voluntarily obey the order to stay at home if possible, saying, "There's a social contract here."

"People, I think, recognize the need to do more and to meet this moment," Newsom said.

California, which has a population of around 40 million, is among the first states to order restrictions statewide. Earlier in the week, Nevada said that all non-essential businesses should close for 30 days.

"We project that roughly 56 percent of our population — 25.5 million people — will be infected with the virus over an eight week period," Newsom wrote in a letter to President Donald Trump dated Wednesday.

Newsom said Thursday evening: "That number in and of itself shouldn't be overly alarming — the vast majority of us, the overwhelming majority of us, won't have symptoms, will be perfectly fine. But there is a subset that we're concerned about" who will be more seriously affected.

The state has more than 870 cases and has had 17 deaths linked to the illness, according to an NBC News count of reported cases, which includes those who have recovered, those who have died and those repatriated to the U.S. from other countries.